1. Field of the Invention
This invention relates to a coupler for microwave energy. In particular, the invention concerns means for coupling a selected mode from one microwave waveguide to another waveguide. A particular application of this invention is in the guidance mechanism of an auto-tracking satellite antenna system in which higher order waveguide modes are used to develop elevational and azimuthal information respecting the position of the boresight axis of the antenna relative to the signal source.
When an incident wave is received by an antenna, the output level of the communications signal is maximum when the antenna points directly toward a point signal source. On the other hand, higher order modes are excited in the waveguide when the boresight axis of the antenna feed is not in line with the point source. For example, the dominant mode in a circular waveguide is the TE.sub.11 mode. The higher order mode TE.sub.21 and TE.sub.21 * are orthogonal modes which may be used to generate error signals for use in a servo system or for tracking a beam. In addition, the TE.sub.01 mode and the TM.sub.01 mode as well as conjugate modes may be used with the dominant TE.sub.11 mode for this purpose.
Mode couplers which generate higher order modes can be classified into three categories. In the first category, herein designated the traveling wave coupler, a series of apertures are provided along the length of a common wall of juxtaposed waveguides. A mode is generated by using a coupled wave mechanism in which the E-vectors add as a wave passes successive holes. In a second category, herein designated as a geometric coupler, modes are generated using a particular geometrical shape for a single aperture or set of apertures. In the third category designated a resonant coupler, modes are generated by the development of standing waves in a resonant cavity which is tuned to the resonant frequency of the mode. A resonant coupler can only be used for narrow-band frequency operation.
The present invention is of the type known as a traveling wave coupler.
2. Description of the Prior Art
U.S. Pat. No. 3,918,010 to Marchalot describes an optimized rectangular-to-circular wave-guide coupler of the traveling wave type. In the Marchalot patent, a metallic tongue is disposed within a circular waveguide opposing a line of equally spaced holes of equal diameter. The metallic tongue is formed in a manner to attenuate propagation modes other than TE.sub.01 or TE.sub.02. Isolation of about 20 dB is claimed.
A number of geometric couplers are known. For example, U.S. Pat. No. 3,566,309 to Ajioka and U.S. Pat. No. 4,246,583 to Profera et al.
A number of resonant couplers are also known. For example, U.S. Pat. No. 3,369,197 to Giger et al., U.S. Pat. No. 3,646,481 to Den and U.S. Pat. No. 2,963,663 to Marcatili.
Not to be confused with mode couplers which generate higher order modes are mode couplers which generate a dominant mode. Such devices are disclosed in U.S. Pat. No. 3,922,621 to Gruner and U.S. Pat. No. 3,731,235 to Ditullio et al.
A further patent of interest is U.S. Pat. No. 3,569,870 to Foldes. This patent describes a feed system, but it does not employ mode coupling.
Without prejudice, reference is made to articles by Y. H. Choung, K. R. Goudey, and L. G. Bryans, entitled "Theory and Design of Ku-band TE.sub.21 -mode Coupler", IEEE Transactions on Microwave Theory and Techniques, November 1982; and Y. Choung, K. Kilburg, and T. Smith, "Ku-band Tracking Feed for Earth Terminal Operation", 1982 APS Symposium Digest, Antennas and Propagation, Vol. II, May 24-28, 1982 (IEEE Antennas and Propagation Society) 82CH17383-0. These articles describe elements of the present invention.